The long term goal of our research is to characterize the cellular and molecular mechanisms that endow an organism with its ability to adapt to changes in the external environment. In particular, we propose to determine how the marine mollusc Aplysia californica makes a switch between two types of related, but distinctly different, rhythmic behaviors. We have developed a hypothetical model that postulates that when this switch is made there are changes in the activity of two unit CPGs'. Mechanoafferent neurons produce the necessary changes in one of the 'unit CPGs'; proprioceptive neurons produce the change in the second ~unit CPGs~. Our work is, therefore, relevant to studies of plasticity in any rhythmic behavior that must accommodate changes in the external environment in a coordinated fashion. Additionally, experiments are proposed that will characterize specific mechanisms that insure that sensory-induced changes in the firing patterns of the feeding circuitry; (a) do not conflict with the ongoing motor pattern, and (b) are exerted in a manner that is related to the specific characteristics of the stimulus inducing the behavioral change. For example, in preliminary experiments we have described an interesting mechanism that is likely to play a role in insuring that sensory-induced changes in motor programs are phase specific. Our data suggest that sensory neurons are rhythmically depolarized during motor programs, and that this depolarization facilitates transmission at synaptic contacts with motor neurons. We propose to test a novel hypothesis that postulates that CPG-induced depolarizations impact sensory neurons in two ways; (1) the release of the primary neurotransmitter is enhanced, and (2) the release of peptide cotransmitters is enhanced or evoked. We propose, therefore, to characterize a novel role for peptide cotransmission in a neural circuit undergoing a transformation from one rhythmic output to another. This data will help guide future studies of the role of neuromodulation in circuit selection, and may provide insights into dysfunctions of the nervous system that occur when cognitive processes necessary for the proper choice of behavior are perturbed, as may be the case with obsessions and compulsions.